Wireless communications systems use multiple access protocols, such as time-division multiple access (TDMA), frequency-division multiple access (FDMA), code-division multiple access (CDMA), and space-division multiple access (SDMA) protocols, to enable wireless communications between base transceiver stations and multiple subscriber units. Typically, a wireless communications system includes multiple base transceiver stations that are spaced to create subscriber cells. Subscriber units, which may include mobile or fixed units, exchange information between a nearby base transceiver station over a dedicated radio frequency.
First generation wireless communications systems utilize single antenna transceivers to exchange information between a base transceiver station and a subscriber unit. While wireless communications systems with single-antenna transceivers work well, they are subject to channel degradation from, for example, multipath fading and/or signal interference. Next generation wireless communications system have been developed which utilize multiple spatially separated antennas at the base transceiver station and/or the subscriber unit to transmit a subscriber datastream over multiple paths. Transmitting a subscriber datastream between a base transceiver station and a subscriber unit over multiple paths is referred to as transmission diversity. Because the transmission antennas are spatially separated, the receiving device receives diverse signals that can be processed to reduce multipath fading and to suppress interfering signals.
While transmission diversity involves transmitting a subscriber datastream over multiple paths, each of the transmitted datastreams carries the same information. New wireless communications systems have been developed that utilize multiple spatially separated antennas at the base transceiver stations and the subscriber units to multiplex transmissions in order to increase the bit rates between the base transceiver stations and the subscriber units. The technique, known as spatial multiplexing, is described in U.S. Pat. No. 6,067,290, which is assigned to the assignee of the present invention.
Although spatial multiplexing works well to increase the bit rate between base transceiver stations and subscriber units that support spatial multiplexing, not all base transceiver stations and subscriber units support spatial multiplexing. In addition, even when a base transceiver station and a subscriber unit both support spatial multiplexing, it may not be appropriate to utilize spatial multiplexing for every transmission. For example, transmission conditions may be such that spatial multiplexing provides poor results, thereby eliminating the possibility of an increased bit rate that is provided by spatial multiplexing.
In view of the fact that not all base transceiver stations and subscriber units in wireless communications systems support spatial multiplexing and in view of the fact that spatial multiplexing may not always be the most effective transmission technique between base transceiver stations and subscriber units even when both devices support spatial multiplexing, what is needed is a wireless communications system and method that adapts its mode of operation between spatial multiplexing and non-spatial multiplexing in response to various transmission-specific variables.